This invention relates to a frequency detection system, especially a system capable of measuring frequency shift in a signal which may have undergone a phase shift.
One example of the application of such a detection system is in an electronic identification system comprising an interrogator and one or more non-contact transponders. A signal at a first frequency is radiated by the interrogator, and the non-contact transponder returns in response a second signal at a slightly different frequency; the frequency difference may be small in magnitude and must be measured very accurately. The frequency difference identifies a particular transponder.
In some electronic identification systems, for example a system used to control a locked door, or in a supermarket check-out to allow automatic billing of selected goods, the distance between the interrogator and the transponder can be controlled, and the system designed to operate accordingly. Often the transponders are arranged to alter the Q factor of the system in a manner characteristic of each transponder, and the change in Q factor is measured. Often the change in Q factor is effected by shorting a coil in the transponder, and such a system is described in our co-pending UK Patent Application No. 9505350.0 filed on 16th Mar. 1995.
If however the distance between the transponder and the interrogator is increased, then the influence of a shorted coil may be too small to give a significant change in Q factor. To increase the operational distance of the system, a high frequency back-signalling arrangement is required, as set out in our UK Patent Applications 9508600.5 filed on the 27th Apr. 1995 and 9511085.4 filed on the 1st Jun. 1995.
A disadvantage of the arrangement set out in these applications is that the transponder requires an increased energy level which may not be supplied easily.
It is a first object of the invention to provide a frequency detection system based on a digital filter having a novel input arrangement.
It is a second object of the invention to provide an electrical identification system, based on a shorted-coil transponder system, and capable of operating over substantial distances without the need for a high power source.
According to the invention a bandpass filter comprises:
first supply means to supply a first cyclical analogue signal at a centre frequency f.sub.1 ; PA1 divider means to divide the centre frequency by an integer N and supply the frequency divided signal to a digital filter as the system clock of said digital filter; PA1 second supply means to supply a second cyclical analogue signal at a frequency to be measured f.sub.2 which differs from the centre frequency by a small amount; PA1 delay means to delay the second signal by N cycles; PA1 subtraction means to receive the current second signal S.sub.c and the delayed second signal S.sub.p and to provide any difference to the digital filter as a difference signal; PA1 whereby the digital filter provides an output signal in which the centre frequency is suppressed and which is related to the frequency of the second signal. PA1 antenna means to receive a second signal at a frequency differing by a small amount from the centre frequency; PA1 delay means to delay the second signal by N cycles where N is an integer; PA1 subtraction means to receive the second signal and the delayed second signal and to provide any difference as a difference signal to a digital filter; and PA1 frequency division means to divide the centre frequency signal by an integer N and supply the divided frequency to the digital filter as the system clock of said digital filter; PA1 whereby the output of the digital filter has the centre frequency suppressed and is related to the frequency of the second signal.
The delay means typically comprises a dynamic store, and the integer N is typically 20.
Also according to the invention an interrogator for an electronic detection system comprises a first oscillator arranged to transmit a cyclical analogue interrogation signal at a centre frequency;
Optionally the divider means is a phase-locked loop comprising a phase discriminator supplied by the first oscillator, a voltage controlled oscillator, and a divide unit.